In order to lower the nitrogen oxide emissions in diesel vehicles, retreatment of the exhaust gases by what is known as selective catalytic reduction (SCR) increasingly takes place. With the aid of this technology, commercial vehicles can satisfy the Euro-V standard and passenger cars can comply with the very stringent American exhaust gas standards. In selective catalytic reduction, a chemical reaction takes place at the SCR catalytic converter. This reaction is selective, that is to say nitrogen oxides (NO, NO2) are preferentially reduced, while undesirable secondary reactions, such as, for example, the oxidation of sulphur dioxide into sulphur trioxide, are largely suppressed.
For the reduction of nitrogen oxides, ammonia (NH3) is required which is admixed to the exhaust gas. The products of the reaction are water (H2O) and nitrogen (N2). The ammonia required for the SCR reaction is not used directly, that is to say in pure form, in vehicles, but in the form of a 32.5% aqueous urea solution which is designated uniformly by industry as AdBlue. The composition is stipulated in DIN 70070. This aqueous solution is injected, upstream of the SCR catalytic converter, into the exhaust tract where ammonia and CO2 arise from the urea solution as a result of a hydrolysis reaction. The ammonia thus generated can react with the nitrogen oxides in the exhaust gas in the SCR catalytic converter at an appropriate temperature.
The consumption of urea/water solution amounts to about 2 to 8% of the diesel fuel used, depending on the untreated emission of the engine. A corresponding accessory tank having a corresponding volume is therefore provided for the urea solution in vehicles. The filler neck of this accessory tank is closed by means of a tank closure assembly. This includes a closing body of cap-like design which is screwed onto the filler neck. In the closing body, a receptacle is provided for a sealing element which serves for sealing off an annular gap between the filler neck and the closing body when the closing body is screwed onto the filler neck.
In the tank closure assemblies known hitherto, the closing body is produced with the internal thread and with the receptacle for the sealing element as an injection moulding. However, this is difficult to manufacture, since the inner contour has, particularly in the region of the receptacle for the sealing element, undercuts, the production of which requires split moulds. This leads on the finished product to mould separation edges and production inaccuracies which may cause leaks during use.